Hollow shafts are required in many industries. Most manufacturers of hollow shafts use boring techniques to form the annular bores of the hollow shafts.
Many industries including the wind energy industry use hollow shafts. These hollow shafts which are used in gear boxes are typically subjected to high static and dynamic loads.
For many decades these components have been manufactured using conventional manufacturing process where continuously cast (concast) bloom or cast ingot is subjected to forging process which results into forged bars followed by proof machining, heat treatment and final machining.
However, Conventional manufacturing method results into large machining time and poor yield. This results into substantial raw material wastage during mass production.
The existing method of manufacturing these parts are using cast polygonal/round ingot or bloom with no or minimal hot working as input. The machining includes OD turning, finishing, facing; deep hole drilling; boring, counter boring followed by heat treatment and final machining.
In a nutshell, the existing manufacturing method is a combination of forming a forged bar from a concast bloom or an as-cast ingot followed by machining. It has been found that the existing process results in about 50% utilization of material (thereby leading to about 50% wastage of material from the time of forming a bloom/ingot to making of the proof machined part).
The manufacturing of hollow shaft related inventions are reported in prior art. It is mentioned hereunder.
United States patent US20030221514A1 discloses a method of manufacturing hollow shaft with power transmission members such as cam of camshaft, crown gears of gear shaft or journals of crankshaft disposed one behind the other in an axial direction, the power transmission members being configured to be at least hollow.
U.S. Pat. No. 6,062,116 discloses method of manufacturing hollow shaft and mandrel for holding cylindrical hollow shaft blank. Hollow shaft is manufactured by defining a through hole axially in a shaft blank to produce a cylindrical hollow shaft blank, inserting a mandrel in the through hole of the hollow shaft blank, holding opposite ends of the mandrel concentrically with the through hole, and rotating the hollow shaft blank about its own axis while cutting an outer circumferential surface of at least one end of the hollow shaft blank concentrically with the through hole to form a reference outer circumferential surface on the at least one end of the hollow shaft blank.
U.S. Pat. No. 4,425,774 discloses an apparatus for extrusion forging essentially comprises a double action hydraulic press composed of an inner ram. Extrusion forging of a billet is accomplished by placing the billet in a container and inserting a punch into the container thereby forcing the billet through the die. After the extrusion forging, the punch is raised to a prescribed level and held there and the container is subsequently raised. Since the non-deformed part of the billet is attached fast to the container, the rising container drags the extruded part of the billet out of the die and, at the same time, brings the non-deformed part of the billet into powerful collision with the punch and consequently knocks the extrusion forged product out of the container.
U.S. Pat. No. 4,803,880 discloses a process for forging hollow elongated articles from super alloys and titanium alloys. The process employs preconditioned material which has low strength and high ductility. The process is performed in a forging press and has an initial step which converts a preform into an intermediate shape by press motion which produces radial outward work piece flow. Press punch geometry is then changed and the operation continues with radial inward flow about a mandrel.
U.S. Pat. No. 7,360,388B2 discloses a hollow stepped article is formed from a solid blank to reduce the material cost, and cracking is prevented in a stepped portion of large diameter when a portion of the blank is deformed by its radial expansion. A hollow stepped shaft is formed by holding an upper and a lower part axially of a solid rod-like blank with an upper and a lower die, respectively, which have a stepped recess of large diameter in a region where they are opposed to each other; compressing the blank from both its axially opposite sides with an upper and a lower punch each of which is smaller in diameter than the blank, thereby extruding the blank so that an axial hollow is formed therein about its axis in each of its upper and lower parts and that a portion of the blank opposed to the stepped recess of large diameter expands in diameter and deforms into that recess while leaving a solid plug-like portion between the punches; and thereafter further compressively moving one of the punches to shear the solid plug-like portion and force it out of the blank, whereby the blank is formed with a stepped portion of large diameter by radially expanding deformation in a region intermediate between its opposed ends or at one of these ends and with a continuous axial hollow about its axis.
The critical review of the prior art reveals following technological gaps such as, lack of manufacturing process modification, lack of machining work optimization, lack of material utilization, lack of virtual manufacturing technique implementation for process optimization of hollow shafts.
With conventional manufacturing method, which results into large machining time and poor yield, substantial raw material is thus wasted during mass production of such components.
Moreover, existing methods, which involve either no reduction or minimal reduction by hot working, do not result in superior material properties required for deployment in high power applications.
Another important limitation of the existing methods arises from the fact that the hollow shafts under consideration have steps in the bore involving multiple diameters. It is known that such multiple-stepped bores can be produced by machining only and therefore cannot provide continuous grain flow along the contour of bore. This is one of the main reasons why the hollow shafts produced using the current methods lack superior grain flow characteristics and mechanical properties.
There is therefore a need to provide alternative methods of designing and manufacturing hollow shafts, involving near-net shape forging, which would result in superior grain flow characteristics and mechanical properties and result in more effective utilisation of raw material.